The present invention relates to a method for rendering and/or evaluating a surface quality of a workpiece based on program data used for machining. These program data include a set of points describing the surface, wherein the set of points describes points along the path of space curves. The invention also relates to a method for optimizing the surface quality.
High-speed machining or high-speed cutting (HCS) has recently become more widespread in milling operations. The development of new technologies, such as high frequency spindles, modern cutting materials and highly dynamical, digital feed drives in numerically controlled machine tools and robots has resulted in increased use of HSC machining.
In particular, the construction of tools and molds has experienced significant advantages, such as reduced machining time, lower machining costs and shorter throughput times as compared to conventional machining of materials.
The high precision of HSC installations can also eliminate the need for manual finishing, in particular when free-form surfaces are milled.
The free-form surfaces are typically curved three-dimensional surfaces, for example fenders of an automobile or turbine blades. When these workpieces are milled, a very high measurement accuracy and surface quality is generally required.
However, the HSC method can also cause problems in certain applications. For example, in the construction of molds, the surface quality may no longer be guaranteed due to unpredictable errors. The precise, highly dynamical drives of the HSC machines appear to cause more errors than conventional drives.
The underlying cause could be determined quickly by comparing two workpieces milled on different machines. The machine that produces a poor surface quality would then be responsible for generating the machining errors. Such behavior, however, can generally not be determined. There are obviously certain factors which are influenced by different data processing or construction of these machines.
Older, less dynamic drives react rather sluggishly to the (drive) control input variables. This smoothes out small irregularities or flaws of the control input variables. As a result, rather soft, homogeneous surface structures are obtained which, however, may have greater measurement tolerances (smoothing).
However, a HSC machining process with its precise, highly dynamic drives can transfer the control variables more exactly. Smaller flaws which were previously smoothed out then become increasingly visible. Rough, inhomogeneous (hard outlined) surface structures are the result.
This processing method can achieve a higher accuracy at the expense of sometimes unacceptable surface quality.
The present invention combines the advantages of HSC machining with the high quality of conventional machining. This is accomplished by determining the underlying causes for the inadequate quality of HSC machining and by proposing improvements.
The path from a virtual model to the milled workpiece will hereafter be described into form of a process chain. This process chain is a linear sequence of individual decoupled process steps. It is therefore necessary to determine the causes for the inadequate surface quality across the entire process chain.
The process chain can be subdivided into four larger main areas. FIG. 2 shows such a simplified process chain.
Three well-defined points (CAD model, NC data, control variables) exist within the process chain. The analysis of the NC input data (NC program) is an object of the present invention.
Except for a few special cases, the process chain depicted in FIG. 2 with the processing paths (CAD constructionxe2x86x92NC controllerxe2x86x92Machine Drives) is presently almost exclusively used. Processing is typically preformed using linear sets. However, several manufacturers of control systems have since several years been able to process spline-based workpiece models without prior conversion into linear sets. In the future, the conversion could be eliminated entirely (by processing the CAD data without converting them first in the NC controller). Since an intermediate step is eliminated or at least significantly simplified, possible conversion errors are also reduced. However, until then, conventional processing methods will most likely be used to ensure compatibility.
The surface quality can be improved by intentionally intervening in the control variables. However, this intervention either prolongs machining times or replaces the old errors with new errors. Moreover, optimizing the control variables is quite costly due to the complexity of the control path. An analysis of the NC input data (workpiece data) is therefore a next step for reducing errors. Recognizing the errors is a first step in their elimination.
Until now, surface quality has been rendered by providing a surface grid of the set of points describing the surface and by then evaluating the rendition. This rendition view, however, provides only a limited view of the surface quality, because it is difficult to recognize the large number of the surface quality features of complex CNC program data.
It would therefore be desirable and advantageous to provide an improved process, which obviates prior art shortcomings and which can be used to analyze the surface quality of a workpiece, that is still to be manufactured, based on CNC program data before the workpiece is actually machined, and to thereby quickly and easily recognize possible error locations or in accuracies. In addition, an effective method for visualizing the determined surface quality should also be provided.
According to one aspect of the present invention, a method for rendering and/or evaluating the surface quality of a workpiece based on program data used for machining includes determining and rendering the associated normal vectors for a plurality of adjacent points along the machining path, wherein regions with a high surface quality are indicated by normal vectors that are oriented essentially in the same direction, whereas flaws in the resulting surface are indicated by normal vectors pointing in different directions.
Such normal vectors are preferably determined by forming two vectors from three consecutive path points and arranging the normal vector of a center path point as a vector product perpendicular on a plane spanned by the two vectors, whereby the orientation of the normal vector relative to a side of the plane is selected depending on the direction of curvature of the space curve at the corresponding path point.
According to another feature of the present invention, so-called angle-bisecting vectors can be employed instead of the normal vectors. The corresponding angle-bisecting vectors are determined and rendered for a plurality of adjacent path points. Regions with a high surface quality are indicated by angle-bisecting vectors pointing essentially in the same direction, whereas flaws in the resulting surface are indicated by angle-bisecting vectors pointing in different directions. Preferably, such angle-bisecting vectors can be determined by forming two vectors from three consecutive path points and arranging the normal vector of a center path point as a vector product perpendicular on a plane spanned by the two vectors, and by rotating the normal vector into the plane by an angle of 90xc2x0, so that the angle-bisecting vector is located at half the angle between the two vectors.
According to another feature of the present invention, all determined normal vectors of the path points may be rendered with their center located at one point. Conclusions regarding the surface quality can be drawn based on the distribution of the normal vectors, in that regions of high surface quality are indicated by the essentially overlapping normal vectors, whereas flaws of the resulting surface are indicated by a scatter of the normal vectors in different directions.
Advantageously, the end points of the normal vectors having an identical length and being centered at an initial point are projected in three-dimensional space onto a spherical surface having the radius of the normal vector, and regions of flaws of the resulting surface are indicated by regions with a large number of such end points.
According to another feature of the invention, the angles between corresponding normal vectors of adjacent path point can be determined and regions with a high surface quality can be indicated by comparatively small angles between adjacent normal vectors, whereas flaws in the resulting surface quality are indicated by relatively large angles and/or sudden changes between adjacent angles. Suitably, a tolerance threshold can be defined which is advantageously selected between 10xc2x0 and 25xc2x0. Angles between adjacent normal vectors below this threshold value are assumed to be small angles and angles above the threshold value are assumed to be comparatively large angles.
For effective visualization, the path points associated with adjacent normal vectors having comparatively large angles can be advantageously specially marked. Visualization of the analyzed surface quality can be improved further by the invention by rendering a normal vector in form of an extended surface that extends to one of the adjacent path points on one or both sides of the path axis along the path. The so produced pseudo-surfaces provide an observer with an improved view of the position of the normal vectors or the angle-bisecting vectors that are used for evaluating the surface quality. Visualization can be further improved by marking regions with flaws of the resulting surface by coloring the respective points or lines or surfaces.
In addition to analyzing the NC input data of exemplary freeform surfaces as described above, the present invention is able to not only recognize errors (mathematically analyzed and visualized), but also correct those errors.
This is achieved by a method for measuring and optimizing the surface quality by analyzing the surface according to the invention and manipulating the underlying CNC program data until most or all normal vectors or angle-bisecting vectors on the three-dimensional path point in the same direction. Suitably, the CNC program data can be manipulated by changing the original data points of the set. Alternatively or in addition, the CNC program data can also be manipulated by generating additional data points, in particular by performing a new scan of the three-dimensional path.
In particular, the original data points can be altered by smoothing the three-dimensional path, for example by a linear regression over several adjacent path points, as long as the space curve can be reduced on at least one plane. Alternatively, the three-dimensional path can be smoothed by a two-dimensional compensation spline extending over several adjacent path points.
Alternatively, the three-dimensional path can be smoothed by a three-dimensional compensation spline extending over several adjacent path points. Optionally, the data resolution of the path points of the space curve can be increased.
The present invention is able to realize the following advantages:
a very simple and clear rendition of possible error sources in parts programs
possibility for a simple comparison between an original and an optimized parts program
the effects of control components (e.g., a data compressor) on the path of a processing machine and therefore on the possible surface quality of a workpiece can be effectively viewed and compared
based on the analyzed error sources and/or flaws, the CNC program data can be optimized automatically or manually after inspection by an operator.